"We had a rash of these things at the end of March and beginning of April last year," he said, saying the robberies generally were carried out by two men. "Then they disappeared awhile. Then we started hearing about (similar robberies) in San Jose and San Rafael and now apparently they're back here."

Marovich said there have been at least 26 victims in San Francisco "and a lot of others that are probably unreported." He added, "The word in the Tenderloin was that cops were ripping off people, and that's not true."

The latest thefts came at 2 p.m. Saturday, as a man was robbed of several hundred dollars at a bus stop in a Bernal Heights neighborhood, and 90 minutes later when three men were robbed at a bus stop in Colma.

The usual modus operandi of the thieves is to hit people waiting for buses, but the thieves have other methods, too, Marovich said.

"I'm pretty sure they're the same guys who tried to pull over a female friend of mine on Gough Street by flashing their lights and flashing a badge." The woman did not stop.

Usually, the thieves pull up in an older, big car -- the type that are called "cool cars" by the undercover cops who use them. One of the thieves will get out, flash a badge and demand to know whether the victim is doing or dealing drugs.

He then asks for the victim's wallet, turns his back while using a cell phone to purportedly check on the victim's identification and extracts any cash in the wallet. He then lets the victim go and drives off, Marovich said.

The fact that only one of the men gets out of the car is a tipoff, Marovich said, because real undercover cops almost always get out of their cars together. "If you're dealing with a drug dealer, these guys are armed and you're not going to go to him alone," he said. "If you're dealing with a real suspect and you're going to take somebody on, both of you get out."

Marovich said the robbers prey mostly on Latinos, adding: "They tend to be afraid of police. They're used to police confronting them like that in those countries down there."

He said anyone approached by someone claiming to be a cop should demand identification. Undercover cops have a photo identification and a badge, he said, and are willing to show them.

Victims should try to get a license plate number, Marovich said, and immediately call 911 or a local police station.